<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        /*
          *: :hover  :link  :active  :visited  lvha
          标志性符号：冒号（:）
        */

        .box{
            width: 200px;
            height: 200px;
            border: 1px solid #000;
        }

        .box:hover{
            border-radius: 50%;
            background-color: red;
        }

    </style>
</head>
<body>
    <div class="box">

    </div>
</body>
</html>